Appendix I: Stability of Sexual Chromosets

Stabilization theory claims that the karyotype of a sexual chromoset remains stable because, relative to other members of a population otherwise composed of individuals with fully paired karyotypes, structural heterozygotes are at a severe reproductive disadvantage. For if an alteration in chromosome structure (structural rearrangement) did arise by mutation in a sexual chromoset, it would do so in a single gamete. When that gamete combined with another gamete to form a zygote, the individual that developed from that zygote would have one chromosome of the usual type paired with one showing the new structural arrangement and would therefore be structurally heterozygous. Such an individual would typically be far less fertile than those members of the population with fully paired karyotypes.

Whether a particular arrangement is, or is not, advantageous (from a reproductive standpoint) depends, then, not only upon the intrinsic genetic content of the affected chromosome(s), but upon whether the arrangement is common or rare in the population at large. In the absence of hybridization, new structural arrangements are extremely rare. As long as the original structural arrangement is the type found in the vast majority of the population, individuals having the new arrangement will be at a strong reproductive disadvantage. Because a new structural heterozygote would be 1) of low fertility and 2) would be exceedingly unlikely to find a mate having the same rearrangement because a new arrangement would be excessively rare—in fact, unique. Low fertility means there will be few offspring. The inability to find a “matching” mate means progeny will either be 1) homozygous for the old arrangement or 2) again heterozygous and so, again, of low fertility. Under such circumstances it is in no way likely that a homozygote for the new arrangement would ever arise, but even if it did, it would again face the same problem of finding a matching mate (and so would almost certainly produce heterozygous offspring of low fertility). Moreover, new arrangements arising in a non-hybridizing population would bear no new genes of any kind, let alone advantageous ones. So there would be no improvement in selective advantage. There would therefore be no selection for the new arrangement to help it get established in the population. Strong selection would be necessary to overcome the marked adverse effect of unpaired chromosomes on fertility. In consequence, it is not easy to see how the karyotype of one chromoset would be converted with time into the karyotype of a new chromoset, if the changes had to take place in a gradually evolving, isolated population (Key 1968). That is, there will be powerful selection for karyotypic stability.

This argument can be converted to the language of population genetics and take advantage of an established result: the original form of the chromosome is an “allele present at high frequency." The newly arising structural arrangement is an "allele present at very low frequency." The heterozygote has a lower fitness than either of the two possible homozygotes. The situation just described is termed underdominant selection. With underdominant selection, an allele initially present at an extremely low frequency will be eliminated from the population. New structural arrangements will therefore be eliminated from a non-hybridizing population.

However, in intrachromoset hybridization, structural heterozygotes are often produced in millions, generation after generation. These heterozygotes, and their hybrid offspring, are spatially concentrated in the hybrid zone. Because they are spatially concentrated, the inbreeding among the hybrids is increased, which in turn increases the chances of producing small, local groups of individuals that are structurally homozygous for a new arrangement (and so can breed among themselves to stabilize the new arrangement). Such hybrids may possess new, advantageous gene combinations not present in either pure parent. If a karyotype specifies a new hybrid chromoset at an advantage relative to its two parents, spatial, stochastic computer simulations show the new karyotype can become established in the population despite the reproductive disadvantages inherent in structural heterozygosity, and can rapidly replace the parental karyotype. On a geological timescale, this process of replacement would appear almost instantaneous (the hybrid zone between the new type an its parents would spread as a front, passing over, in just a few generations, any particular geographic site sampled by a paleontologist). The observed pattern would be one of long-term stability followed by an abrupt transition to the new type—the pattern actually observed in the fossil record for a wide variety of fossil forms. When, however, this same computer simulation was altered so that 1) all individuals were identical (i.e., so that no hybridization was assumed) and 2) structural rearrangements were allowed to arise in the population at realistic rates, no new chromosets whatsoever were ever generated.